This afternoon my wife's grandfather gave me an old CVA .45cal cannon. Based on my research it is an 'Old Ironsides' model that was sold in the 70's & 80's as a kit. It was loaded with a charge of powder and wadding so I carefully removed the wadding with a pick and scrapped out the old powder. The cannon is actually rifled and once everything was clean I found that the pitting wasn't too severe.

Anyway, I want to fire this thing, but I'm not sure what kind of black powder to use. Should I use a certain size flake or just whatever I can get my hands on? Pyrodex?

It will shoot .440 size lead balls so I may pick up a box of them if I can find them locally.

Thanks for any help you can give me.

As I recall the barrels were steel and fairly thick, at least as thick as the cylinder and barrel on a .44 caliber pistol like the .44 cal version of the Colt 1851 Navy.  In that regard, you could potentially treat it like a small caliber pistol or rifle, but only if it has proof marks indicating it would take similar pressures.

In a properly proofed black powder firearm, a safe starting load for a muzzle loading rifle is a charge in grains (measured by volume) equal to the caliber, so theoretically 44 grains would be ok for your .440 caliber cannon.   However, I have no idea whether the cannon has any proof marks, so in absence of that, I'd cut the charge in half and consider that the max - about 15-20 grains would be a good range for charges.  You could probably go heavier, but in a short cannon barrel, there is not much point as you won't get any more velocity, just more recoil from the heaver weight of the charge being expelled from the barrel.

I'd stay with black powder if you can find it, as a) it tends to generate less pressure than a similar charge (measured by volume, not weight) of black powder, b) pyrtodex has a higher ignition temperature, so it's more prone to a failure to fire with cannon fuse, and c) it has much harder fouling than black powder.

In general FFFg is used on small caliber rifles and pistols (.45 caliber or less), so you could use FFFg, but I'd also use FFg.  

The important thing is to ensure there is no airspace between the powder and the ball, as that causes a pressure spike and can put a bulge/ring in the barrel.

Given that the canon is small, it can be very difficult to get a tightly patched ball seated, so use a fairly thin patch, or just seat the ball without a patch and use a wad of paper rammed on top to hold the ball against the powder, so it won't roll forward before it fires (like you would with a smooth bore).

For a small bore like .44, use 3 F powder.   If you can get fuse for it, do it.  Fuses are more fun as they give you time to stand back and watch.

Thanks for the help guys! I found some fuse online and I'm checking with my local gun shops for FFFg black powder.

I cleaned all the surface rust off the barrel and am in the process of painting it now so that I will have a protective coat.

This is approximately a pistol barrel, so use a pistol load

15-20 grainsof either FFFg or FFg.

Well, I've checked with every gun shop in my county and a couple outside of my county and nobody seems to carry black powder. I talked to my father-in-law, who has an inline muzzleloader and he had a pound of Pyrodex.

I've read that Pyrodex does not work well in mini cannons, but it's the only thing I can get my hands on at the moment.

Any experience with Pyrodex in a mini cannon?

DakotaFAL noted all the reasons to NOT use Pyrodex above. I would not mess with it.
Personally, I'd go ahead and order a dozen pounds of Goex FFG and FFFG from Grafs, and throw in some percussion caps to make the Hazmat fee worthwhile. Of course, you'll need to get a muzzleloading rifle or pistol to use up that powder, but you're gonna do it anyway once you shoot the cannon and stop giggling.
Resistance is useless.

I regularly used Goex 3f powder - 40 grains.
I bought mine when I was still high school.  It shoots fine without patches.  These little canons are tough.  I overcharged it a number of times, the only ill effect was that it would recoil back several feet.
I still have it, sits in my living room.  A good way to use it as a blank cannon. Is to just stuff paper over a compressed charge of powder.